Sparkover stabilizing means for an ungraded surge voltage arrester

ABSTRACT

An ungraded surge voltage arrester having a plurality of series connected sparkgaps is provided with sparkover stabilizing means that are effective to raise the minimum power frequency sparkover level of the arrester without significantly changing its maximum impulse sparkover voltage. The stabilizing means includes a tapered arrangement of different length sparkgaps in combination with a sparkgap preionizer mounted to ionize the sparkgap having the shortest gap length. Maximum length sparkgaps are disposed adjacent the high voltage terminal of the arrester.

United States Patent [191 Sakshaug SPARKOVER STABILIZING MEANS FOR ANUNGRADED SURGE VOLTAGE ARRESTER [75] Inventor: Eugene C. Sakshaug,Lanesborough,

Mass.

[73] Assignee: General Electric Company, Pittsfield, Mass. 22 Filed:Jan. 26, 1973" [211 Appl. No.: 326,790

[52] US. Cl. 317/70, 315/36 {51] Int. Cl. H02h 9/06 [58] Field of Search317/70, 69; 315/36 [56] References Cited UNITED STATES PATENTS 2,032,5663/1936 Earle 317/70 X 2,179,297 11/1939 Johnson...

3.259.780 7/1966 Stetson 315/36 X 1 1 June 25, 1974 3/1970 Osterhout315/36 X 10/1971 Osterhout et al. 317/70 Primary Examiner-James D.Trammell Attorney, Agent, or Firm-F. X. Doyle; Vale P. Myles; V. R.Ulbrich l 5 ABSTRACT An ungraded surge voltage arrester having aplurality of series connected sparkgaps is provided with sparkoverstabilizing means that are effective to raise the minimum powerfrequency sparkover level of the arrester without significantly changingits maximum impulse sparkover voltage The stabilizing means includes atapered arrangement of different length sparkgaps in combination with asparkgap preionizer mounted to ionize the sparkgap having the shortestgap length. Maximum length sparkgaps are disposed adjacent the highvoltage terminal of the arrester.

11 Claims, 10 Drawing Figures PATENTEU JUN 25 I974 sum 1 0M SHEU 2 BF 4LIIIEV GROUND V HOUSING [El/67b PATENTED JUN 25 1974 SHEET 3 OF 4 flllllll |l w=ll1l T L T lllll |\l SPARKOVER STABILIZING MEANS FOR ANUNGRADED SURGE VOLTAGE ARRESTER BACKGROUND 1. Field of the InventionThis invention relates to an improved surge voltage arrester, and moreparticularly, to a means for stabilizing the sparkover level of surgevoltage arresters that are operated in environments that contaminatetheir insulating housings.

2. Description of Prior Art Modern surge voltage arresters of the typeused to protect the insulation and apparatus on electric powertransmission and distribution systems usually comprise a plurality ofsparkgaps electrically connected in series with one or more blocks ofnonlinear resistance electrical valve material that are housed within aweather resistant housing such as, glazed ceramic. Often the operatingenvironment of such arresters are such that the exterior surface of thearrester housings become s'everely contaminated during their normaloperating life. Due to the high voltage gradient that is applied acrossthe arrester housings this type of surface contamination can operate tocause erratic sparkover of the arresters if some means is not providedto stabilize their sparkover voltage. The problems associated withcontamination of arrester housings have long been recognized and a rangeof various solutions have been proposed. For example, it is commonpractice in the field of high voltage arresters to provide a voltagegrading network electrically connected in parallel with the plurality ofsparkgaps used in station-type surge voltage arresters to uniformlydistribute the voltage gradient across each of the sparkgaps. Suchvoltage grading circuits have proven to be very successful incounteracting the problem of erratic sparkover due to contamination ofthe arrester housing; however, the cost of voltage grading circuits issufficiently high to suggest the use of alternative means, if possible,to solve this problem; particularly in distribution type surge voltagearresters that would not otherwise require the use of a voltage gradingcircuit to assure proper operation of the arrester.

In the field of ungraded distribution-type surge voltage arresters it isgenerally known that when a plurality of sparkgaps are seriallyconnected within the arrester between a line terminal and a groundterminal the sparkgaps nearest to the line terminal will be subjected tothe greatest voltage gradient. This phenomenon is due primarily to thefact that all of the leakage current in the sparkgap assembly must flowthrough the line terminal sparkgap; whereas lesser amounts of leakagecurrent flow through each of the other gaps due to the capacitivecoupling between the respective sparkgaps and the insulating arresterhousing. Accordingly, if some form of sparkgap preionization is used tostabilize the sparkover level of such a surge voltage arrester, thepreionization means is usually mounted adjacent the sparkgap that isconnected to the line terminal because it will normally be the first gapto sparkover.

It is also known by those familiar with surge voltage arresters that thesparkover level of an ungraded, multisparkgap arrester can be raised byincreasing the gap length of the sparkgap, or sparkgaps, next adjacentto the arrrester line terminal. An example of such a surge voltagearrester is shown in US. Pat. No. 2,179,297 Johnson, which issued onNov. 7, 1939. Another,

somewhat similar, prior art approach to the problem of erratic surgevoltage arresters sparkover due to contamination of the arrester housingis shown in U.S. Pat. No. 2,688,715 Vorts et al., which issued on Sept.7, 1954. In the arrester arrangement disclosed by Vorts et al., a largenumber of sparkgaps is arranged immediately adjacent to an arrester lineterminal while a smaller number of sparkgaps is disposed closer to theground terminal of the arrester thereby to at least partially counteractthe inherent voltage distribution of the arrester due to leakagecurrents flowing through the sparkgap assemblies and the capacitivecouplings to the arrester housing and to ground. One major shortcomingof such prior art attempts at counteracting the ef fects of arresterhousing contamination is that they fail to provide a stable, relativelylow impulse sparkover level while maintaining powerfrequency and impulsesparkovers that do not fall below a desired minimum when the arresterhousing is contaminated. As pointed out above, it is common practice tostabilize the impulse sparkover level of ungraded distribution arrestersby providing a sparkgap preionizer mounted in operative relationship tothe sparkgap that is connected to the line terminal of the arrester.However, when the gap length of the spark gap directly connected to theline terminal is increased to such a point that the arrester minimumsparkover does not fall below the desired point when the arresterhousing is contaminated, the maximum impuse sparkover of the arrester isincreased to an undesirable level. Although it is generally known tomount sparkgap preionizing means adjacent sparkgaps that are remote froma line terminal sparkgap in a series discharge circuit, in the mannershown for example, in US. Pat. No. 3,169,208 Harrington, which issuedFeb. 9, 1965, it is practically necessary from an economic standpoint torely on the line voltage as a source of preionizer energy indistribution-type lightning arresters. Thus, relatively complexpreionizing circuits as that disclosed in the Harrington patent are notgenerally suited for application in distribution class surge voltagearresters of the type commercially utilized to protect electric powerdistribution systems.

SUMMARY OF THE INVENTION It is a general object of this invention toprovide a new and improved ungraded surge voltage arrester that has astable impulse voltage sparkover level and that is highly resistant toerratic sparkover at voltages lower than its power frequency rating,even when the arrester housing becomes contaminated.

It is a further object of this invention to provide a new and improvedsurge voltage arrester, having the abovedescribed characteristics, whichis economical to construct and simple to manufacture.

In a preferred form of the invention, a surge voltage arrester isprovided comprising a hollow elongated insulating housing having a lineterminal at one end and a ground terminal at the opposite end, with aplurality of sparkgaps mounted in spaced-apart, serial relationshipinside the housing. A non-linear electrical valve serving as a currentlimiting resistor is electrically connected in series with thesparkgaps, and the sparkgaps and valve are electrically connectedbetween the line and ground terminals to form an overvoltage dischargepath. The sparkgap immediately adjacent and electrically connected tothe line terminal has a gap length greater than the gap length of anyother sparkgap in the arrester. The sparkgap immediately adjacent andelectrically connected to the ground terminal has a sparkgap preionizermounted in operative relationship with it to stabilize the impulsesparkover level of the arrester.

Alternative embodiments of the invention disclosed herein deal withthelengths of other sparkgaps than the sparkgap adjacent the line terminal.Generally, in these embodiments, sparkgaps are positioned in a taperedarrangement decreasing from a maximum gap length at the top, or lineterminal of the arrester to the sparkgap having the shortest gap lengthat the ground terminal end of the arrester.

BRIEF DESCRIPTION OF THE DRAWINGS A more complete understanding of theinvention may be had by referring to the accompanying detaileddescription and drawings in which:

FIG. 1 is a sectioned side view of a surge voltage arrester illustratinga plurality of sparkgaps arranged with various different gap lengths, incombination with a preionizer mounted adjacent the ground terminal endof the arrester pursuant to the invention;

FIG. 2 is a schematic cirucit representation of the surge voltagearrester depicted in FIG. 1;

FIG. 3 is a sectioned side view of a surge voltage arrester shown toapproximately true scale;

FIG. 4 is a schematic representation of the capacitance network of thearrester housing and internal parts;

FIGS. 5 and 6 are graphs of voltage distribution versus housing lengthfor the arrester shown in FIG. 3; and

FIGS. 7, 8, 9 and 10 are schematic representations of furtherembodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1, there isshown a surge voltage arrester 1 having an elongated hollow insulatinghousing 10, made of a suitable material such as glazed porcelain, whichprovides a strong, weather resistant protection for the arresterelements mounted in it. At the outset, it should be understood thatwell-known, conventional materials and structural configurations may beused in constructing an arrester embodying the teaching of the presentinvention. For example, housing, electrode and valve materials andconfigurations similar to those disclosed in U.S. Pat. No. 3,614,523Stetson et al., which issued on Oct. I9, 1971 and is assigned to thesame assignee as the present invention, may be employed when arranged asdisclosed herein.

An electrically conductive, metal line terminal 11 at the upper end ofthe housing is adapted to be connected to an electrical power lineconductor 50 as shown, thereby to operatively couple the arrester tosuch a source of high voltage. A similar conventional ground terminal 12is mounted in watertight relationship over the lower end of the housing10 and, as shown, is suitably adapted to be connected to groundpotential.

A plurality of brass electrodes, 13 through 18, are mounted,respectively, in spaced-apart relationship between disc shapedinsulators a, 20b, 20c, 20d and 20e, which have apertures 21a, 21b, 21c,21d, and 212 formed therein. A plurality of sparkgaps are formed betweenthe raised, or boss-like, portions, of each pair of electrodes that faceone another through one of the apertures. These sparkgaps havepredetermined gap lengths that are established by the thickness of theinsulating discs. For example, the pair of electrodes 13 and 14 ismaintained in a spaced-apart relationship by the insulator 20a and formsa sparkgap 22 that is the sparkgap closest to line terminal 11.

A plurality of nonlinear electrical valves 24a, 24b and 240 arepositioned within the insulating housing as shown. The valves may beformed of any conventional non-linear negative resistance material, suchas molded discs of granular silicon carbide. As is well-known in theart, the purpose of the non-linear valves is to limit power followcurrent while affording a low resistance discharge path for high currentsurges.

A compression spring 28 mechanically supports the plurality of pairs ofelectrodes and the valve elements in series between the line terminal 11and the ground terminal 12. As is well known, a shunting strap 28a maybe connected across the spring 28 to maintain the low impedance of thesurge current discharge path between the terminals 11 and 12 througheach of the sparkgaps and the valves. Thus, when an overvoltage ofpredetermined magnitude causes the sparkgaps to sparkover, an electricalcurrent path is established from the protected line conductor 50,through the line terminal 11, the valve elements, the sparkgaps, and theground terminal 12 to ground potential.

Pursuant to the invention, a preionizer 30 is located in an operativerelationship with the sparkgap 23, which is the sparkgap closest to thearrester ground terminal 12. In the preferred embodiment beingdescribed, the preionizer 30 comprises a resilient metal electrode 30ahaving a pointed contact 30b mounted on it for contacting the insulatordisc 20a. As shown, the preionizer electrode 30a is electricallyconnected to the electrode 18 so it is energized by the electric fieldgradient existing between the electrode and the disc 20e, when voltageis applied to arrester terminal 1 1. A slot 31 in disc 20e extends fromthe aperture 21e for the sparkgap 23 to receive the preionizer 30.Various types of self-energized preionizers have been found to besuitable for stabilizing the impulse sparkover voltage of the sparkgap(23) closest to ground terminal 12. Thus, alternative preionizerconfigurations may be used with other embodiments of the invention. Forexample, capacitively coupled preionizer of the type disclosed in US.Pat. No. 3,504,226 Stetson, which issued Mar. 31, 1970 is suitable forthis purpose. It is important in selecting a preionizer to select onethat will respond to the relatively low voltage gradient across thesparkgap 23 closest to ground terminal 12, so that the sparkoverstabilizing objective of the invention can be attained.

FIG. 2 is a schematic diagram of the surge voltage arrester described inFIG. 1 showing the line terminal 1 1; the ground terminal 12; a singlenon-linear valve 24, which is intended to reference all the valveelements of the arrester; the electrodes 13, l4, 15, 16, 17 and 18; andthe preionizer30. The sparkgap 22 closest to the line terminal 11 isformed by the pair of electrodes 13 and 14, and the pair of electrodes17 and 18 form the sparkgap 23 closest to the ground terminal. Thus FIG.2 clearly illustrates the overvoltage discharge path from the lineterminal 11 through the series sparkgaps to the ground terminal 12.

FIGS. 3, 4, 5 and 6 graphically depict the adverse effect created by thepresence of moisture or contamination on the surface of housing 10, aswell as showing how the present invention overcomes this adverse effectto secure consistent, stable sparkover. FIG. 4 is a schematic diagramshowing the capacitance network of the arrester. Capacitances a througha represent the inherent capacitance of the insulating housing.Capacitances c1, c2, and 03 represent the capacitance from the housingto ground. Capacitances b through bl7 represent the capacitances fromthe internal parts of the arrester to the housing, and capacitances d1through d7 represent the capacitances across the arrester gaps. None ofthe capacitance values are large, but capacitances d are larger than theothers. While the capacitances a, b, c and d are shown as discretecapacitors for the sake of clarity in the following discussion, it iswell understood that the capacitance is actually distributed rather thanlumped at discrete points.

Under clean and dry conditions, curve 35a of FIG. 5 gives theapproximate housing voltage without the internal parts, at the referencepoints shown in FIG. 3. The voltage gradient is highest near the top ofthe arrester because the upper capacitances must carry the current forthe capacitances to ground as well as the current through the lowercapacitances of the housing. For instance, capacitance a3 must carry thecurrent through a4 as well as the current through cl. Therefore, thevoltage drop across the section of housing having capacitance a3 isgreater than the voltage drop across the section of housing havingcapacitance a4. The higher voltage gradient near the top of the housingis shown by the steeper slope of the curve 35a between terminal 11, the0 point in the housing, and point 1 than between points 2 and 3.

The voltage on the internal parts is illustrated by curve 38 of FIG. 5.The voltage at point 1 is very nearly the same as the voltage atterminal 11 because the valve disc 24a is an extremely low impedancecompared to the impedance of the capacitance network. Similarly, thevoltage'at point 2 is almost exactly the same as the voltage of theground terminal because of the low impedance of valve discs 24b and 240and spring 28. The potential of the housing near point 3 is also groundbecause the hanger bracket is grounded, as shown in FIG. 3. Almost allof the arrester voltage appears across the arrester gaps between points1 and 2.

When the complete arrester including housing and internal parts isconnected to a source of high voltage as shown in FIGS. 1 or 3 thehousing voltage becomes approximately as shown by curve 35 of FIG. 5.The shape of the voltage changes from that given by 35a because ofcurrent flow through capacitances b between the housing and the internalparts. Current flowing in capacitances b, b1, b2 etc. is larger than thecurrent in capacitances c to ground. For this reason, the currentthrough a3 is less than the current through a4, and the gradient at theupper end of the porcelain housing is decreased. Similarly the gradientat the lower end of the housing is decreased.

Curve 38 between points 1 and 2 is not a straightline. The slope nearpoint 1 is greater than the slope midway between points 1 and 2 becausegap capacitance d1 must carry the current through b8 in addition to thecurrent through d2. Similarly, capacitance d7 carries current from bothbl2 and d6 so that the voltage across the bottom gap is greater than thevoltage across the gap above it. The slope of curve 38 does not deviatevery much from a straightline because capacitances d are larger thancapacitances b, but as shown previously, the housing voltage is changedby the internal parts because the capacitances b between internal partsand housing are greater than the capacitances between housing andground. The direction of current flow in capacitances b is not the sameat the top of the gap column as it is at the bottom of the column. Asillustrated in FIG. 5, the voltage on the internal parts at point 1 (seecurve 38) is higher than the voltage of the housing at point 1 (seecurve 35). Assuming a positive line voltage, current flows from the gapstoward the housing through capacitances b7, b8, b9. The voltage of thegaps near point 2 is less than the voltage of the housing (compare curve38 with curve 35 in FIG. 5) and current flows from the housing towardthe gaps through capacitances bl l, b12, bl3.

When the housing is coated with a conducting contaminant, the voltagegradient along the arrester length between terminal 1 1 and the groundedbracket tends to be uniform as shown by curve 35b of FIG. 6. Because thecurrent through the conducting contaminant is usually in the order ofmilliamperes, while the steady current through the distributedcapacitances is in the order of microamperes, the voltage gradient ofthe housing is affected very little by the internal parts when thearrester is contaminated. The housing capacitances a through a20 are noteffective in determining the housing voltage because of the lowimpedance of the conducting layer. The current through the capacitancesb near the ends of the gap column is increased because the voltagedifference between the housing and the ends of the gap column isincreased as shown by the difference between the curves 38 and 35 ofFIG. 5 and curves 38 and 35b of FIG. 6. Because of this increase incurrent, the voltage gradient across the end gaps is increased and thevoltage of the internal parts becomes as shown by curve 35d of FIG. 6.Assuming a positive half cycle of line voltage, the current through d1,is the sum of the current through d2 and b8. Similarly, the currentthrough d7 is the sum of the current flowing in d6 and M2.

As the layer of contaminant begins to dry, the housing voltage may beginto assume a shape such as given by curve 350 of FIG. 6. Under suchconditions, the voltage across the gap column will be as illustrated bycurve 35c of FIG. 6. Comparing curves 35c and 352 it can be seen thatthe voltage of any point of the gap column is then equal to or greaterthan the housing voltage opposite that point. Therefore, the current incapacitances b7, b8, etc. from the gap column to the housing flows inthe same direction in all capacitances. The current in capacitance d1 isthen equal to the current flowing in d2 plus b8, the current in d2 isequal to the current in d3 plus b9, and a disproportionate share ofvoltage appears across the upper gaps while the voltage on the lower gapremains normal.

From the above description, it will be clear that FIGS. 3, 4, 5 and 6essentially disclose the voltage and current distribution within and onthe housing of a surge arrestor. The housing lengths show in FIGS. 5 and6 are obviously the various points on the housing in FIGS. 3 and 4. Aswill be understood, the curves of FIG. 5 are generally for the clean anddry condition of the housing while the curves of FIG. 6 are generallyfor the contaminated condition of the housing. However, curve 38 whichillustrates the voltage on the internal parts is included in bothFIGURES as is curve 35b, which is the voltage gradient across thehousing when coated with conducting contaminants. A comparison of thecurves, as above discussed, shows that the voltage gradient of the topgaps is greater than the gradient over the lower gaps.

Because the voltage gradient on the top gaps is increased bycontamination of the arrester housing, the uppermost sparkgap 22 in thepresent invention is formed to have a greater gap length than any othersparkgap in the arrester. The greater gap length of sparkgap 22 enablesit to withstand more voltage without sparkover.

The voltage gradient across the sparkgap 23 does not change greatlyunder contaminated conditions. Accordingly, pursuant to the objectivesof the present invention, the gap length of sparkgap 23 may be decreasedslightly to make it possible to cause this gap to sparkover in a stablemanner by suitably preionizing it with the preionizer 30 as explainedabove. In fact, it has been demonstrated that by use of the uniquetapered gap length arrangement combined with a preionized bottomsparkgap, as taught by the present invention, it is possible to raisethe random sparkover voltage of a given arrester with a given level ofsurface contamination, under applied line voltage conditions, by 35 to40 percent, while maintaining the impulse sparkover rating of thearrester essentially constant within a very stable and narrow range ofvoltages.

Some-alternative embodiments of the invention are schematicallyillustrated in FIGS. 7, 8, 9 and 10. It is to be understood that theseschematic diagrams represent actual surge voltage arrester structuresthat may be somewhat similar to the structure shown in FIG. 1. In allthe schematic representations, the sparkgap 22 closest to the lineterminal 11 has a greater gap length than any of the other sparkgaps ofthe arrester for the purposes of the invention, as explained above and apreionizer 30 is located in operative relationship with the sparkgap 23closest to the ground terminal 12.

In modern surge voltage arresters, it is common practice to utilize aplurality of substantially identical sparkgaps, each of which have asimilar sparkgap length, such as a length of 60 mils. Unlike suchconventional arresters, according to the present invention the gaplengths of different sparkgaps are changed; for example, the sparkgap 22closest to the line terminal has a length in the range of 85 to 95 mils,while sparkgap 23 closest to the ground terminal has a length in therange of 45 to 55 mils in the embodiments of the invention shown inFIGS. 1, 2 and 7.

In FIG. 8, between the uppermost and lowermost sparkgaps are apredetermined number of intermediate sparkgaps, shown for example bysparkgaps 40 and 41. The number of intermediate sparkgaps may vary, buteach intermediate sparkgap must be less in length than the gap length ofsparkgap 22 and greater in gap length than the length of sparkgap 23. Inaddition, the intermediate sparkgaps 40 and 41 may be respectively equalin length, or of different lengths. A typical example of this situationwould be equal gap length intermediate sparkgaps in the range fromgreater than 55 to 65 mils.

In FIG. 9, the sparkgaps 43, 44, 45 and 46 comprise a plurality ofintermediate sparkgaps. In this embodiment the intermediate sparkgapsinclude a first group of intermediate sparkgaps adjacent the sparkgap 22closest to line terminal 1 l and a second group of intermediatesparkgaps adjacent the sparkgap 23. The first group comprises sparkgaps43 and 44, and the second group comprises sparkgaps and 46. FIG. 9illustrates two sparkgaps in each group but a greater or lesser numbermay be employed in each group. The sparkgaps comprising the first groupare respectively equal in length; the sparkgaps of the second group arerespectively equal in length and the gap lengths of the sparkgaps in thefirst group is greater than the gap lengths of the sparkgaps in thesecond group. The first and second groups might, for example, havesparkgap lengths as follows: The sparkgaps 43 and 44 comprising thefirst group have equal gap lengths in the range from greater than 75 to85 mils; and the sparkgaps 45 and 46 comprising the second group haveequal lengths in the range from greater than to mils.

A final embodiment is illustrated in FIG. 10 which includes theintermediate sparkgaps 43, 44, 45, 46, 47 and 48. Just as in FIG. 9, thesparkgaps 43 and 44 comprise a first group and the sparkgaps 45 and 46comprise a second group. A third group of intermediate sparkgaps shown,for example, by the sparkgaps 47 and 48 is located between the first andsecond groups. The sparkgaps of the third group are respectively equalin gap length, less in length than the gap lengths of the sparkgaps inthe first group, and greater in gap length than the sparkgaps of thesecond group. The sparkgap lengths of the embodiment in FIG. 10includes, for example, the gap lengths described for the correspondingsparkgaps in FIG. 9 while the sparkgaps of the third group have equallengths in the range from greater than 65 to mils.

Although certain embodiments of the invention have been shown anddescribed, those skilled in the art will perceive changes andmodifications without departing from the invention, and it is intendedby the appended claims to cover all such modifications and changes asfall within the true spirit and scope of thisinvention.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

l. A surge voltage arrester, comprising an elongated hollow insulatinghousing, a line terminal and a ground terminal respectively mountedadjoining opposite ends of said housing, a plurality of pairs ofelectrodes, each of said pairs of electrodes being mounted within saidhousing in spaced apart relationship to define a sparkgap therebetweenwhich is ungraded by voltage grading means, at least one nonlinearelectrical valve, and means for electrically connecting in series saidplurality of pairs of electrodes, said valve and said terminals therebyto form an overvoltage discharge path between said terminals througheach of the sparkgaps and said valve; including the improvement wherein:

the sparkgap closest to said line terminal has a gap lengthsubstantially greater than the gap length of the sparkgap closest tosaid ground terminal; in combination with a single sparkgap preionizerin said arrester mounted in operative relationship to ionize thesparkgap adjacent said ground terminal, thereby to stabilize itssparkover voltage, whereby the sparkover voltage of the arrester ismaintained relatively stable even when the insulating housing becomescontaminated thereby enabling an appreciable and erratic leakage currentto flow across the housing surface from the line terminal to the groundterminal.

2. The surge voltage arrester as defined in claim 1 wherein saidsparkgap closest to the ground terminal has a gap length less than thelength of any other sparkgap in the arrester.

3. The surge voltage arrester as defined in claim 2 wherein thesparkgaps include an intermediate sparkgap having a gap length less thanthe gap length of the sparkgap closest to the line terminal and greaterthan the length of the sparkgap closest to the ground terminal.

4. The surge voltage arrester as defined in claim 2 wherein thesparkgaps include a plurality of intermediate sparkgaps between thesparkgaps respectively closest to said terminals, each intermediatesparkgap having a length less than the length of the sparkgap closest tothe line terminal and greater than the gap length of the sparkgapclosest to the ground terminal.

5. The surge voltage arrester as defined in claim 4 wherein saidplurality of intermediate sparkgaps are of equal gap length.

6. The surge voltage arrester as defined in claim 4 wherein saidplurality of intermediate sparkgaps comprises a first and a second groupof intermediate sparkgaps, each group having at least one sparkgap, saidfirst group being adjacent to the sparkgap closest to said line terminaland the second group being adjacent to the sparkgap closest to saidground terminal, each of the sparkgaps of said first group being ofequal gap length, each of the sparkgaps of said second group being ofequal gap length, and the sparkgaps of said first group having a gaplength greater than the gap length of the sparkgaps of said secondgroup.

7. The surge voltage arrester as defined in claim 6 wherein theplurality of intermediate sparkgaps includes a third group of sparkgapsbetween said first and second groups, said third group including atleast one sparkgap, each sparkgap of said third group being of equal gaplength, the gap length of the sparkgaps of said third group being lessthan the gap length of said first group and greater than the gap lengthof the sparkgaps of said second group.

8. The surge voltage arrester as defined in claim 1 wherein the gaplength of the sparkgap closest to said line terminal is in the rangefrom 85 mils to 95 mils.

9. The surge voltage arrester as defined in claim 2 wherein the gaplength of the sparkgap closest to said line terminal is in the rangefrom 85 mils to 95 mils and the gap length of the sparkgap closest tothe ground terminal is in the range from 45 mills to 55 mils.

10. The surge voltage arrester as defined in claim 3 wherein the gaplength of the sparkgap closest to the line terminal is in the range from85 mils to 95 mils and the gap length of the sparkgap closest to theground terminal is in the range from 45 mils to 55 mils.

11. The surge voltage arrester as defined in claim 6 wherein the gaplength of the sparkgap closest to said line terminal is in the rangefrom 85 mils to 95 mils, the gap length of the sparkgap closest to theground tenninal is in the range from 45 mils to 55 mils, theintermediate sparkgaps comprising said first group have a gap length inthe range from greater than mils to mils, and the intermediate sparkgapscomprising said second group have a gap length in the range from 55 milsto 65 mils.

i "UNITEDSTATES PATENT omicE CERTIFICATE OF CO-RRECTIQN Patent NO. :9 IDated JUDGES, 197 4 EUGENE SAKSHAUG Inventofls) It is certified thatetror appears in the abpve-identified patent and that said LettersPatentare hereby corrected as shown below:

"1 Col. 4, .Line 36 "20ashould be 2oe--.

Signed and 'seal ed this 7th day; 'of January 1975.-

(SEAL) I Attest:

McCOY M. GIBSON JR. 'c. MARSHALL DANN Attesting Officer C onupissionerof Patents FORM po-soso (10.69)

. UNITED ATEs PATENT OFFICE CERTIFICATE OF I CO-RRECTIQN I Patent NO.9:9

Dated June'25, 197 4 EUGENE SAKSHAUG Inventofls) It is certified tha terror appears in the above-identified patent and that said Letters P atent are hereby corrected as shown below:

v" Col; Line 36,- "2083 should be 2oe-- Signed and qealegzl this 7th dagof January 1975,

(SEAL) Attest:

McCOY M. GIBSON JR. c. MARSHALL DANN Attesting Officer Comissioner 'ofPatenrs

1. A surge voltage arrester, comprising an elongated hollow insulatinghousing, a line terminal and a ground terminal respectively mountedadjoining opposite ends of said housing, a plurality of pairs ofelectrodes, each of said pairs of electrodes being mounted within saidhousing in spaced apart relationship to define a sparkgap therebetweenwhich is ungraded by voltage grading means, at least one nonlinearelectrical valve, and means for electrically connecting in series saidplurality of pairs of electrodes, said valve and said terminals therebyto form an overvoltage discharge path between said terminals througheach of the sparkgaps and said valve; including the improvement wherein:the sparkgap closest to said line terminal has a gap lengthsubstantially greater than the gap length of the sparkgap closest tosaid ground terminal; in combination with a single sparkgap preionizerin said arrester mounted in operative relationship to ionize thesparkgap adjacent said ground terminal, thereby to stabilize itssparkover voltage, whereby the sparkover voltage of the arrester ismaintained relatively stable even when the insulating housing becomescontaminated thereby enabling an appreciable and erratic leakage currentto flow across the housing surface from the line terminal to the groundterminal.
 2. The surge voltage arrester as defined in claim 1 whereinsaid sparkgap closest to the ground terminal has a gap length less thanthe length of any other sparkgap in the arrester.
 3. The surge voltagearrester as defined in claim 2 wherein the sparkgaps include anintermediate sparkgap having a gap length less than the gap length ofthe sparkgap closest to the line terminal and greater than the length ofthe sparkgap closest to the ground terminal.
 4. The surge voltagearrester as defined in claim 2 wherein the sparkgaps include a pluralityof intermediate sparkgaps between the sparkgaps respectively closest tosaid terminals, each intermediate sparkgap having a length less than thelength of the sparkgap closest to the line terminal and greater than thegap length of the sparkgap closest to the ground terminal.
 5. The surgevoltage arrester as defined in claim 4 wherein said plurality ofintermediate sparkgaps are of equal gap length.
 6. The surge voltagearrester as defined in claim 4 wherein said plurality of intermediatesparkgaps comprises a first and a second group of intermediatesparkgaps, each group having at least one sparkgap, said first groupbeing adjacent to the sparkgap closest to said line terminal and thesecond group being adjacent to the sparkgap closest to said groundterminal, each of the sparkgaps of said first group being of equal gaplength, each of the sparkgaps of said second group being of equal gaplength, and the sparkgaps of said first group having a gap lengthgreater than the gap length of the sparkgaps of said second group. 7.The surge voltage arrester as defined in claim 6 wherein the pluralityof intermediate sparkgaps includes a third group of sparkgaps betweensaid first and second groups, said third group including at least onesparkgap, each sparkgap of said third group being of equal gap length,the gap length of the sparkgaps of said third group being less than thegap length of said first group and greater than the gap length of thesparkgaps of said second group.
 8. The surge voltage arrester as definedin claim 1 wherein the gap length of the sparkgap closest to said lineterminal is in the range from 85 mils to 95 mils.
 9. The surge voltagearrester as defined in claim 2 wherein the gap length of the sparkgapclosest to said line terminal is in the range from 85 mils to 95 milsand the gap length of the sparkgap closest to the ground terminal is inthe range from 45 mills to 55 mils.
 10. The surge voltage arreSter asdefined in claim 3 wherein the gap length of the sparkgap closest to theline terminal is in the range from 85 mils to 95 mils and the gap lengthof the sparkgap closest to the ground terminal is in the range from 45mils to 55 mils.
 11. The surge voltage arrester as defined in claim 6wherein the gap length of the sparkgap closest to said line terminal isin the range from 85 mils to 95 mils, the gap length of the sparkgapclosest to the ground terminal is in the range from 45 mils to 55 mils,the intermediate sparkgaps comprising said first group have a gap lengthin the range from greater than 75 mils to 85 mils, and the intermediatesparkgaps comprising said second group have a gap length in the rangefrom 55 mils to 65 mils.